(1) Field of the Invention
The present invention relates to a packet transfer apparatus and, more particularly, to a packet transfer apparatus connecting a plurality of broadcast segments allowing movement of communication terminals among the segments.
(2) Description of the Related Art
In an IP network, for example, a plurality of broadcast segments constructed by the Ethernet (registered trademark) or the like are connected to each other via a packet transfer apparatus called an L3 switch or a router. In this case, each of the broadcast segments corresponds to a network unit generally called a subnetwork.
The address of each subnetwork is expressed as, for example, “192.168.0.0/24”. A numerical sequence “192.168.0.0” expresses the value of an address part of 32 bits and the value of address bits is described in a decimal of 0 to 255 on a byte unit basis The 32-bit address part is comprised of a first bit group indicative of the address of the subnetwork and a second bit group indicative of a host address. The numerical value “24” after the slant mark denotes the number of bits of a subnet mask.
In the example, the value “192.168.0” of upper 24 bits (three bytes) of the address part specified by the subnet mask denotes the subnetwork address and the lowest one byte indicates the host address. The host address is used as an identifier of each of terminals belonging to a subnetwork. When an IP packet is received from a subnetwork, the router recognizes the destination subnetwork from the subnetwork address part of the destination IP address included in an IP header and transfers the received packet to an output port corresponding to a segment connected to the destination subnetwork.
To each terminal, an IP address having, in its upper-bits part, the same address value as that of the subnetwork to which the terminal is connected is assigned. When the terminal moves from a home subnetwork to another subnetwork, the IP address of the terminal has to be changed. For example, in the case where the Ethernet is used for a data link layer, the subnetwork corresponds to a broadcast segment of the Ethernet. Therefore, when a terminal moves from one broadcast segment to another broadcast segment, the IP address of the terminal has to be changed.
Usually, the subnetwork is defined in consideration of requirements on network management such as separation of traffic and security. For example, in many cases, a subnetwork is defined for each room or floor of an office building and used as an independent broadcast segment. In recent years, laptop personal computers and mobile terminals being small, light, and easy-to-carry are spread as terminals of an IP network. Further, as a wireless LAN is becoming practically used, it is becoming easier to connect a mobile terminal to a network at a movement destination of a terminal user.
Under such circumstances, there is an increasing demand for communication service such that each terminal is allowed to move among subnetworks (or broadcast segments) without changing the terminal IP address and the terminal user can easily use a network at a destination. To facilitate use of a network at a destination of a terminal user, there are known, for example, following network configurations.
(1) A network configuration in which the scale of a subnetwork (broadcast segment) is enlarged and the range of a terminal movement, for example, the whole area of an office building is covered by one subnetwork. In this configuration, a plurality of radio access points are located in the same subnetwork.
According to the network configuration, since each terminal can be connected to the same subnetwork at every destination, it is unnecessary to change the terminal IP address and the IP address of a default router. In the configuration, however, since all of terminals are connected to the same broadcast segment, traffic is concentrated on the same network and it causes a problem such that an available bandwidth for each terminal becomes short. In addition, broadcast traffic reaches all of terminals, so that assurance of security is insufficient.
(2) A network configuration employing a mobile IP specified by RFC2002 of IETF. In a mobile IP, a subnetwork serving a mobile terminal as a home link is provided with a home agent (HA) function and a link at a destination of the mobile terminal movement is provided with a foreign agent (FA) function. When a terminal moves to a subnetwork out of the home link, the relation between the home address of the terminal and a care-of-address (CoA) obtained from the present subnetwork at the destination is notified to the home agent HA (registration of the terminal position).
A source terminal of IP packets designates a home address of a receiver terminal in a destination address field of each of the IP packets to be sent. The IP packets are captured by the home agent HA. The home agent HA encapsulates the IP packet with an IP header having a care-of-address corresponding to the home address as a destination address, and transmits the encapsulated IP packet to a subnetwork where the receiver terminal exists. The encapsulated packet is decapsulated by the foreign agent FA, and the resultant packet is transferred to the receiver terminal. In the mobile IP, therefore, each terminal has to have a mobile IP function of performing position registration from a visited subnetwork where the terminal exists now to the home agent HA.
(3) A network configuration of a host address routing type as proposed in, for example, Japanese Patent Publication No. 2002-135289, in which a router detected movement of a terminal broadcasts the IP address of the moved terminal to the other router, thereby changing the contents of a routing table of each router in association with movement of terminals.
According to the configuration, each terminal can always use the same IP address. However, each of the routers in a network has to hold and update path information of all of the terminals, and the load for path control on the network increases. This configuration, therefore, requires to suppress the changes in the network configuration as much as possible and to suppress the load of the path control by, for example, performing path control on a subnetwork unit basis.